Idle and Large Ambient Temperature Change
 

During the past week the temperatures in the upper Midwest have fallen from the high 80s into the low 50s, with night time record lows near 40F. Just as this happened, I noticed that my idle went a bit funny. It idles normally at about 880rpm for about 30 seconds and then drops below 800, stumbles and slowly rises back up to just about 800. Could the temperature change (and higher air density) do this? Does it take the brain (ECU, not my brain) a while to adjust to large temperature changes? Or should I be looking for other causes (ISV, air flow meter, mixture, vacuum leak, etc.)? :confused:

When was the last time the O2 sensor was replaced?

20K miles ago

Next time you go to start the car, disconnect the sensor, both plugs. Report the findings. The ICV can be cleaned with some carb cleaner. In general, when the temperature is cold, the heater plug on the O2 sensor may take a few seconds to become effective which is why when starting cold the idle will raise and drop to normal.

If it's dropping below normal (880 rom's) you may want to have the fuel mixture and idle re-set. Try the above before having to take it in for adjustments....